The present invention relates to well drilling and servicing fluids utilized in producing and injecting formations and the removal of filter cake deposited by the fluids.
Drilling and servicing fluids may be used to produce hydrocarbon formations penetrated by well bores. The drilling fluids may be utilized when well bores are drilled into producing or injecting formations to minimize damage to the permeability of the formations and their ability to produce hydrocarbons and or accept injected fluids or gas. Servicing fluids may be utilized when completion operations are conducted in formations and when conducting work-over operations in the formations. The drilling and servicing fluids may deposit filter cake on the walls of the well bore within a formation, which may prevent the drilling and servicing fluids from being lost into the formation and may prevent solids from entering the porosities of the formation. After the drilling or servicing of a formation has been completed, the filter cake may be removed prior to placing the formation on production or injection.
The removal of the filter cake may be accomplished by including an acid soluble bridging agent in the drilling or servicing fluid for bridging over the formation pores. The filter cake formed by the drilling or servicing fluid, which may include the bridging agent and a polymeric suspending agent, may be contacted with a strongly acidic solution, and the acidic solution may be allowed to remain in contact with the filter cake for a period of time sufficient to degrade the filter cake. In spite of efforts to avoid it, the strongly acidic solution may corrode metallic surfaces and completion equipment, such as sand screens, leading to their early failure, and may present handling concerns. The acidic solution may also be incompatible with the permeable formation and may cause damage thereto.
Water soluble particulate solid bridging agents may also be used in drilling or servicing fluids, and the filter cake containing the water soluble bridging agent may be contacted with an aqueous salt solution which may be undersaturated with respect to the water soluble bridging particles. However, such undersaturated aqueous solutions may require a relatively long period of time to dissolve the particles, at least partially due to the polymeric suspending agents included in the drilling or servicing fluids. That is, the polymer or polymers present in the filter cake may shield the water soluble bridging particles from the aqueous solution.
Many of the problems associated with filter cake removal may be present in both cased and open hole completions. For open hole completions, the removal of the bridging agent may lead to uncontrolled fluid losses into the formation along the completion interval. Such losses may not only result in damage to the formation due to intrusion of the treatment fluids, but also may result in limited break through of the filter cake clean-up fluid in only a small portion of the interval leading to large volumes of fluid escaping through the limited break. The fluid loss may lead to large portions of the interval and filter cake remaining untreated. The filter cake left behind due to the diversion of the filter cake clean-up fluid may result in a decreased flow rate through some sections, ultimately leading to a decrease in production from or injection into the well.